Storage of solar and other sources of renewable electricity may be enabled by the endothermic production of chemical fuels such as H2 or reduced carbon-containing compounds via the electrochemical reduction of H2O or CO2, respectively. In particular, the renewable production of liquid fuels provides a clear route to energy supply and distribution and addresses energy needs associated with transportation, which account for more than 20% of US energy demand. Moreover, liquid fuels are compatible with existing infrastructure for energy supply and distribution. The societal importance and economic value of liquid fuel resources clearly highlights the need for new platforms that enable the sustainable generation of liquid fuels from CO2, and distinguishes CO2 activation and reduction chemistry as a critical area of focus in the fields of renewable energy storage and molecular energy conversion.
An attractive strategy for the synthesis of carbon-based fuels using renewable energy is the marriage of a robust electrocatalyst for CO2 reduction with a photoelectrochemical (PEC) device or a conventional electrolyzer powered by a renewable source of electrical current. Several CO2 reduction products can be targeted via the half reactions shown in equations 1-3. For instance, the direct electrochemical reduction of CO2 to methane or methanol (Eq. 1 and 2) are attractive energy storing reactions, however, the kinetic hurdles associated with these multielectron proton-coupled electron transfer (PCET) reactions are large, which significantly complicates such processes. By contrast, the 2e−/2H+ reduction of CO2 to carbon monoxide (Eq. 3) is another energetically uphill half reaction that delivers a versatile and energy rich commodity chemical. In addition to being useful for the industrial production of methanol, acetic acid and some plastics, CO can be reacted with H2O via the water-gas shift (WGS) reaction to generate H2. This CO/H2 mixture (synthesis gas) can be used to generate synthetic petroleum and liquid fuels using existing Fischer-Tropsch (FT) methods for direct integration into existing energy storage and distribution networks.
Much effort has been devoted to the heterogeneous reduction of CO2 at metallic electrodes with the goal of driving selective formation of CO via Eq. 3. The majority of such studies have been carried out using aqueous electrolytes with tightly controlled pH requirements (pH ˜8.5-10.5). Under aqueous conditions, the standard potentials for the two electron reduction of CO2 to CO is only 0.12 V more negative versus RHE (the reversible hydrogen electrode) than the competing two electron reduction of protons to H2 (Eq. 3 and 4). As such, for the rate of CO2 reduction to outcompete hydrogen evolution at the cathode, the proton availability of the aqueous electrolyte must be minimized. This has historically been accomplished by using concentrated aqueous carbonate or bicarbonate electrolytes. Under such conditions, there are a handful of cathode materials that can drive the conversion of CO2 to CO. However, only noble metals such as Ag and Au have been shown to catalyze this electrochemical reaction with Faradaic Efficiencies (FEs) that are in excess of 80% at ambient pressures. The implementation of Ag and Au cathodes for electrochemical production of CO has been hampered by two distinct factors. Firstly, the exorbitant cost of these noble metals eliminates their practical use on the scale required for alternative fuel synthesis. The second issue concerns the limited current densities associated with CO production at Ag and Au electrodes, which is directly linked to the kinetics of CO2 electrocatalysis at these platforms.CO2+8H++8e−→CH4+2H2O Eo=−0.12 V vs. RHE  (Eq. 1)CO2+6H++6e−→CH3OH+H2O Eo=−0.12 V vs. RHE  (Eq. 2)CO2+2H++2e−→CO+H2O Eo=−0.12 V vs. RHE  (Eq. 3)2H++2e−→H2 Eo=0 V vs. RHE  (Eq. 4)
These limited current densities are a direct consequence of the required basic electrolyte solutions for which the solubility of dissolved CO2 is very low. Several strategies have been employed to combat the inherently low concentration of CO2 at high pH. These include utilization of 3-dimensional and gas diffusion electrodes, elevation of CO2 pressure in the electrolysis cell and use of additives such as ionic liquids (ILs) or organic solvents, which can dramatically improve the solubility of CO2 in the electrolyte solution. Various metal electrodes have been studied for CO2 reduction activity in non-protic solvents, which display excellent CO2 solubility at ambient pressure, such as acetonitrile (MeCN) and dimethylformamide (DMF). Although the hydrogen evolution reaction is highly suppressed under these conditions, the electrochemical reduction of CO2 in organic electrolytes often leads to product mixtures that can include formate, oxalate and glyoxalate in addition to CO. As a result, there are few materials that can catalyze the electrochemical conversion of CO2 to CO in organic catholyte with even modest FEs. Moreover, the few metals that can drive this electrocatalytic process with reasonable current densities do so only upon application of very large overpotentials. The dearth of cost effective systems that can efficiently and selectively drive Eq. 3 highlights the need for new electrode/electrolyte pairings that can promote the electrocatalytic conversion of CO2 to CO at appreciable rate (high current density) and with high Faradaic and energy efficiencies.
Carbon monoxide is a valuable commodity chemical that is required for the production of many other products, including plastics, solvents and acids. It can also be used directly to prepare other valuable reagents such as hydrogen via the industrial Water-Gas-Shift process. Also, carbon monoxide is the principal feedstock for the industrial Fischer-Tropsch process, which allows for the large-scale production of synthetic petroleum.
Carbon dioxide is also a waste product from conventional power plants. Collection and sequestration of carbon dioxide is commonplace. The ability to convert this waste product to a commodity chemical such as carbon monoxide can offset the cost of sequestration and is of interest to current power producers. Moreover, an attractive strategy for storage of renewable energy resources such as solar or wind is electrochemical fuel synthesis from carbon dioxide. This technology has not yet been realized commercially due to the lack of electrode systems capable of driving the conversion of carbon dioxide to fuels or fuel precursors. Thus, it would be advantageous to develop technology which bridges this gap by allowing electricity from a photovoltaic assembly, wind turbine, etc. to be used to drive fuel production.
Another desirable development would be technology that provides the ability to generate carbon monoxide directly from carbon dioxide on a small scale. Carbon monoxide is required for commodity chemical synthesis, which includes some pharmaceuticals and other species that require carbonylation and hydroformylation chemistry. Since carbon monoxide is an expensive and toxic feedstock, the ability to generate small quantities of this chemical on demand allows it to be prepared as needed as opposed to relying on large stockpiles of carbon monoxide produced using conventional methods. This strategy would also reduce costs associated with safety and carbon monoxide use.
The present invention addresses the above-mentioned objectives, among others.